Breathe easy

Christian Rehn of National Ventilation answers your questions about the nuts and bolts of installing MVHR ventilation systems to provide good air quality in increasingly air-tight homes.

With new build properties required to be increasingly airtight to meet Building Regulations Part L – ‘Conservation of fuel & power’, it is important to consider ventilation for your self-build property. But with so much choice on the market, where should you begin?

Why do I need to ventilate my property?

Without modern ventilation included in a new build you will end up with an airtight box which cannot breathe, resulting in stuffy, warm, poor quality indoor air and condensation and mould forming both on the surface and within the fabric of the building. However, by installing effective whole house ventilation you can prevent this by reducing humidity, filtering out pollen and pollutants from outside air and introducing clean, tempered air. This will result in your home having good indoor air quality, making it a healthier place to live.

What type of ventilation should I install?

We recommend mechanical ventilation with heat recovery (MVHR) as the best solution for new build properties because you can control humidity, CO2 and indoor air quality. MVHR is increasingly the ventilation system of choice for new build projects as it is a controlled way of ventilating a whole property.

Do I need a ventilation system design?

Yes! MVHR systems need to be properly designed to ensure you buy a correctly sized unit for your property. If an MVHR unit is oversized or undersized, it won’t provide efficient, effective ventilation. When it comes to designing the system, contractors will need to be familiar with the range of technologies available, as well as having specialist knowledge of the equipment or software programmes to correctly calculate the pressure in the system over a given length of ducting.

This can be quite daunting for contractors who are unfamiliar with ventilation system design. Luckily, there are design services available, such as the free service we offer, which can help self- builders and contractors by designing the ventilation system for their project.

What type of ducting should my contractor use?

We recommend radial ducting, since it is easy and quick to install and minimises air leakage so that the fan performs as it is designed to. Traditional plastic ductwork is arduous to install and is easy to fit incorrectly, resulting in restriction to airflow and undue stress on the system. Semi rigid radial ductwork is a cost effective, simple alternative. It is ideal for self-builds since it’s flexible but robust and is much quicker and easier to install, with a Stanley knife the only real tool needed.

Simple installation means fewer mistakes, resulting in an airtight installation and improved system performance, and a plethora of accessories are available to overcome any issues that may occur. Radial ducting can also be used in Passivhaus projects.

How is the ventilation controlled?

Traditionally MVHR is often boosted using bathroom lighting but not every bathroom visit is for a shower or bath, and so systems can boost unnecessarily. Our preferred option for control is an in-duct ‘humidistat’ – an efficient, cost-effective control method which is fitted in the ducting and triggers the boost airflow rate when humidity rises. A manual switch in the kitchen means the system can be easily boosted when cooking.

Do I need bathroom and kitchen fans?

No. MVHR is a whole house ventilation system which means you won’t need a fan in the bathroom or kitchen. You do not need trickle vents either, and you don’t even need to fit an extracting cooker hood – just fit a recirculating cooker hood since an extractor cooker hood can reduce the efficiency of the MVHR system.

Is MVHR expensive to run?

Not at all. Generally speaking small and medium sized units cost around £20 a year – larger units up to £40, depending on individual tariffs. An MVHR unit fitted with an in-duct humidistat allows the system to run very efficiently since it runs on ‘trickle’ most of the time and only runs on ‘boost’ when needed.

Is maintenance required?

The only maintenance needed is a filter check every six to 12 months to see if they need changing – the MVHR system has a filter indicator which lights up once a filter is dirty. Changing a filter is easy and just involves opening the flaps of the MVHR unit, pulling out the filter, changing it and closing it back up.

Is MVHR noisy?

The key to this is good system design and installation. As long as MVHR has been designed, installed and commissioned correctly it will not be noisy. When on the MVHR unit is so quiet that it should not be heard. On boost it might be heard where it is sited, although boost is only necessary during showers, baths and cooking so it should not disturb anyone.

Christian Rehn is national design and project manager at leading UK-based ventilation manufacturer National Ventilation